Ethylenically-unsaturated compounds that are free-radically curable by either irradiative (UV, electron beam) or thermal means, and that are capable of reducing the viscosity of a second ethylenically-unsaturated resin species for easier coating, casting, molding, and the like, are referred to as "reactive diluents." When the particular method of curing is irradiative, the materials may be known as "radiation-curable diluents" or "radiation-hardenable diluents." Often, the diluents copolymerize with ethylenic unsaturation in the second resin species to effect crosslinking and, the effect most usually sought, hardening or toughening of the resin.
Reactive diluents play a crucial role in binder precursor compositions used in the production of abrasive articles, in that the binder precursor compositions are typically very viscous and conventionally diluted with an appropriate solvent, usually an organic solvent, prior to coating or casting. The use of such solvents is increasingly undesirable from both environmental and human health viewpoints. Reactive diluents provide a means of decreasing the viscosity of the binder precursor composition to the point of convenient coatability without adding undesirable solvents, thus providing 100% solids resin compositions. (As used throughout this disclosure, the term "100% solids resin" means that substantially all, and preferably all of the liquid components of the binder precursor composition are reacted with the other ingredients of the binder precursor composition, i.e., there are substantially no volatile organic compounds which are emitted during the curing of the binder precursor composition.) See, e.g., assignee's U.S. Pat. No. 5,178,646.
To be commercially useful, reactive diluents for binder precursor compositions useful in the abrasives art preferably exhibit a combination of properties: adequate solvency of the resin(s); rapid cure rate; contribution to, or at least no diminution of, the final hardness of the cured system; and, of course, low cost.
A class of aminoplast hard resins particularly useful in the formation of the make coating, size coating, both coatings, or as a backing treatment of a coated abrasive backing, or in fibrous non-woven abrasive products is described in U.S. Pat. Nos. 4,903,440; 5,055,113; and 5,236,472; all of which are incorporated by reference herein. Aminoplast resins have at least one pendant unsaturated group per molecule or oligomer, and therefore may be cured using radiation energy. These unsaturated groups are preferably positioned .alpha.,.beta. with respect to the carbonyl moiety, and can be acrylate, methacrylate or acrylamide type groups. Such resins are obtained in general by the reaction of amino compounds with aldehydes to produce compounds having hydroxyalkyl groups which are further reacted with hydroxyalkyl esters of acrylic or methacrylic acid to form compounds with pendant groups having ethylenic unsaturation positioned .alpha.,.beta. from a carbonyl group (referred to herein as ".alpha.,.beta.-unsaturated carbonyl groups"). In the presence of a suitable initiator, the unsaturated aminoplasts can be cured by either thermal or irradiative means (or a combination thereof) to form a hard, crosslinked binder resin which finds utility in abrasive articles. Many compounds previously used as diluents, such as the alkoxylated polyacrylates, are not effective solvents for aminoplast resins.
Thus, there is a need for reactive diluents which exhibit excellent solubility for aminoplast resins, which are highly reactive to both photochemical and thermal free-radical polymerization, which exhibit low vapor pressures, which exhibit low viscosity at temperatures of about 20.degree. C. and which enhance or, at the least, do not diminish, the hardness of cured resins in which they are used. We have discovered several such diluents.